My MC is a very powerful and old demon he's been around for thousands of years. He was basically a former high ranking military officer and was charged with a mission to attack this country but something happened and his troops were unable to make it but he's kept them alive and is planning to still go through with the attack even though and incredible amount of time has passed. His reasons are for the simple fact that he was given a mission and feels obligated to complete it he also made a promise to his men no matter what he'd see the mission through. Does this sound believable and does it make sense?
It'd makes sense if he's from a culture of honour and there's a bit of backstory that gives cause for his loyalty. He could feel personally indebted or have been conditioned (to the extreme) by his training. Alternatively—what being a demon and all—it could be part of his genetic make up. Much as a cat's is to hunt. I think you just need a little exposition.
I'm confused about the details, because countries don't tend to last for thousands of years. Can he even identify the entity that he's attacking? But that aside, no, it doesn't make sense unless there are added factors that make it make sense. As it is, it makes him sound not quite sentient--like a robot, a computer, or a trained animal.
Yeah the war he fought in was World War Two so the country is still around. For him it's a matter of pride combined with his feeling of obligation to his soldiers who promised no matter what he'd finish the mission.
like those Japanese soldiers who didn't surrender until 1973 or something, its plausible. However there is an issue about why a demon feels loyalty to a mortal cause ?
You also said... ...which implies it was a lot earlier in his thousands of years of existence, so querying whether the country is still around is valid. No, this doesn't make sense. A promise to his men is a stupid reason...they don't want him to see his mission through, they just want to get home alive. If he sees his mission through, there's a good chance that some of them won't make it, and he'll be the one getting the medals, not them. Hirō Onoda (小野田 寛郎, Onoda Hirō?, March 19, 1922 – January 16, 2014) was an Imperial Japanese Army intelligence officer who fought in World War II and was a Japanese holdout who did not surrender in 1945. After Onoda spent 29 years holding out in the Philippines, his former commander traveled from Japan to personally issue orders relieving him from duty in 1974. This isn't the same thing. Onoda wasn't actually engaged in offensive operations, he was just surviving and awaiting the Japanese push to recover lost ground. He didn't feel "obligated to complete a mission".
I assumed that his men had been killed trying to complete the mission and he had promised to honor their memory by finishing what they started. To me that makes sense as a motivation .. what doesn't make sense is why a demon feels that degree of fealty with human soldiers in the first place
He does it because he's something of a noble demon. Over the centuries he's fought in many wars and has been part of many armies he takes war and orders very seriously.
I think I may get where you are trying to go with this, now that ive read all the comments, but I don't think I buy into "noble demon". Noble implies he is loyal to something greater than himself. Demons are, by definition, agents of evil. Therefore, it follows that the thing greater than a demon would be, well, something MORE evil, not a concept like loyalty. Betrayal maybe, and the pain it causes. The terrors of war... yeah, I could see him liking that. But don't see the motivation for him coming from mortal men- his soldiers or otherwise, unless it had to do with all the terrible, painful things he could do to them. If you are trying to make this loyalty to his men his endearing trait, ehh, I just dont buy it. On the other hand, accidentally discovering a flicker of what the mortals call loyalty in his quest to be the worst war demon ever, but brought on by his own selfish devotion to destruction- this seed of "doubt" ultimately unravelling his philosophy of evil? Resulting in a "fallen" demon, left to live out his life as a haunted mortal? Now that I'd read. So, no, I don't think it is believable. Whatever you decide to do with him, my advice would be to dig much, much deeper. And steer far, far from the same ole 'war is hell, and this dude is sooo bad, he just loves it, but love him anyway, because he's a noble fellow." Meh. Just my two cents. Good luck!